core: Include "id" attribute in schema for cbd depends datasource test

This attribute is referenced in order to include a computed value into
another resource, and so it must be present in the schema so that it can
be properly resolved.

terraform/context_plan_test.go

@@ -3729,6 +3729,7 @@ func TestContext2Plan_createBeforeDestroy_depends_datasource(t *testing.T) {
 		DataSources: map[string]*configschema.Block{
 			"aws_vpc": {
 				Attributes: map[string]*configschema.Attribute{
+					"id":  {Type: cty.String, Computed: true},
 					"foo": {Type: cty.Number, Optional: true},
 				},
 			},

terraform/test-fixtures/plan-cbd-depends-datasource/main.tf

@@ -10,5 +10,5 @@ resource "aws_instance" "foo" {
 
 data "aws_vpc" "bar" {
   count = 2
-  foo   = "${count.index}"
+  foo   = count.index
 }